We propose here to investigate the growth and maturation of the primary spermatocyte. Our previous studies have largely ignored this stage, which comprises about one-third of the process of sperm formation. As it is the period during which the molecular stage is set for spermiogenesis, it is of utmost importance that we understand the events taking place there in order to have a comprehensive understanding of sperm development. The other major aspect of this proposal is the genetic dissection of the processes taking place in the testis. This approach will entail examination of genotypes that are incompatible with male fertility. We will attempt to identify the earliest departure from normal development and to characterize the sequelae of the inferred primary lesions of these genotypes. This approach will allow us to define genetically controlled steps in the processes and to infer sequential dependencies of the steps in normal development. Three general classes of male-sterile genotypes will be investigated with both the light microscope and the electron microscope: y-chromosome deficiencies, mutations, and chromosome aberrations. These male sterilizing consitutions are available in abundance owing to our past efforts, and they will now be employed in our investigations of the genetic control of spermatogenesis.